Drop In Mode - Balance 210

Hi, I am looking for some guidance in configuring my Balance 210 in a slightly unusual use case.

Because I am transitioning from dual fixed wireless to fibre in 6 months, I am building up a network that can handle the faster speeds that fibre will bring. I would like to keep my wan bonding running via the Balance 210 but insert a new router between the 210 and my bridged firewall appliance. In this case the bridge firewall appliance likely does not factor into the issue because it is bridged.

At present the system works fine with no multiple NAT issues however I need to know how I would setup the Balance 210 (drop in settings, IP’s, etc) and if I need to change anything on the new router to make this work?

Any assistance would be welcome.